Respiratory viral infections in infants can have devastating effects acutely on airway function, but may also impact the longterm function of the lung in both children and adults. The most common respiratory infection that is the predominant cause of hospitalization in children (>90%) is respiratory syncytial virus (RSV) infection. The initiation of the proper anti-viral responses are mandatory for successfully clearing this pathogen with minimal pathophysiologic responses. In the present proposal we will focus on the earliest immune responses to RSV infection involving the initial activation of toll- like receptors (TLRs) on dendritic cells (DCs) followed by the upregulation of important instructive signals that initiate the acquired immune responses. Recent findings have identified that notch/notch ligand induced activation has a profound role on the activation and differentiation of mature T cells. The upregulation of specific notch ligands on DCs is MyD88-dependent and provides a critical step in mature T cell differentiation. However, little is known about the role of Notch/notch ligand activation pathways for the generation of effective immune responses during virus infections. Our hypothesis for this proprosal is that TLR-mediated notch ligand delta-like 4 is required for the initiation of the appropriate immune response, and without it RSV infection becomes more pathogenic and results in an altered immune environment. These studies will specifically address several novel mechanistic questions by progressing through 3 specific aims that will 1) determine the critical TLR-induced DC activation pathway during RSV infection for anti-viral instructive signals; 2) identify the role of delta- like 4 in the development of RSV-induced immune responses and pulmonary pathology; 3) determine the differential role of plasmacytoid versus conventional DC populations for the expression of delta-like 4 and T cell activation in RSV infection. Together these individual specific aims, which are independent of one another yet clearly integrated, will each address our overall hypothesis. We will investigate these observations mechanistically using a combination of studies in gene knockout mice, specific neutralizing antibodies, and cell transfer experiments along with DC and T lymphocyte isolation. The use of specific novel reagents and advanced techniques will allow our highly integrated group of investigators to specifically target these mechanisms in a logical translational manner.